1. Field of the Invention
The present invention relates to a display device for displaying data, and more particularly to a display device having a ferroelectric liquid crystal panel.
2. Related Background Art
The use of a bistable liquid crystal element has been proposed by Clark and Lagerwall (JP-A- 107216/1981 and U.S. Pat. No. 4,367,924). Ferroelectric liquid crystal having chiral smectic C phase (Sm C *) or H phase (Sm H *) is usually used as the bistable liquid crystal. This liquid crystal has bistable state to an electric field, including a first optically stable state (first orientation state) and a second optically stable state (second orientation state). Accordingly, unlike an optical modulation element used in a TN type liquid crystal, the liquid crystal is oriented in the first optically stable state for one electric field vector, and the liquid crystal is oriented in the second optically stable state for the other electric field vector. The liquid crystal of this type quickly responds to the applied electric field to assume one of the two stable states and maintains the state when the electric field is removed. Many of the problems involved in the TN type element are essentially resolved by making use of the above property.
In the display device which uses the TN type element, the TN type element has no memory function and hence the content of display is not stored in the display panel. Accordingly, no special means for erasing the display content is necessary from a security stand-point of confidential information. On the other hand, in the display panel which uses the bistable ferroelectric liquid crystal, the display content is stored in the display panel. In a transmission type display device which allows observation of the display content by illumination of a back light, the stored display content is not recognized when the back light is turned off, but when the back light is turned on, the stored display content appears. This raises a problem with security of confidential information.
In the liquid crystal display device of this type in which scan electrodes and information electrodes are arranged in a matrix and liquid crystal is filled between the electrodes to form a number of pixels to display the image, a scan signal is sequentially and periodically applied to the scan electrodes while a video signal is applied to the information electrodes in synchronism with the scan signal. In this case, the transfer of the video signal and the selection of the scan electrode are done by at least three signal lines for vertical synchronization signal VD, horizontal synchronization signal and video signal DATA. The vertical synchronization signal VD is produced at a period of one screen (one frame) time, and the horizontal synchronization signal is produced at a constant period (1H period) by at least the number required to scan the horizontal scan electrodes. The VD and HD are always in a fixed relation, that is, in a synchronized relation, and n video signals DATA are transferred in the 1H period, where n is the number of information electrodes.
In the transfer system which uses the three signal lines, a leading scan electrode of the screen is selected at the VD pulse, the scan starts from that scan electrode, and other the scan electrodes are sequentially scanned from the top to the bottom of the screen by the HD pulses. At the same time, the video signal DATA is transferred to the sequentially selected scan electrodes to form one screen. The above operation is repeated 30 times (30 frames) or more per second.
In a large size and multi-pixel display device, the frequencies of VD, HD and DATA are necessarily high if the display panel is driven at higher than 30 frames per second. For example, where the display panel has 400 scan electrodes and it is driven at 30 frames per second, the 1H period corresponds to 80 .mu. seconds.
When the ferroelectric liquid crystal is used as the material of the liquid crystal display panel, there is no known practical ferroelectric liquid crystal material which allows writing (updating) of pulses applied to the scan electrodes at a rate of 80 .mu. seconds per 1H period. If more than 80 .mu. seconds of time is given to the 1H period to apply the pulses so that the writing (updating) of the screen is done by the conventional signal transfer system and drive system, the number of frames is smaller than 30 frames per second. In this case, the scan state is visible by human beings and the quality of the displayed image deteriorates. Further, since the scan electrodes are sequentially scanned and all information electrodes have the video signal always applied in synchronism with the scan signal, the power consumption is high.